Spherical particles have increasing applications in industrial processes. Spherical particles provide good flowability, low surface area and hence a minimum of surface oxide, and efficient packing. Applications for relatively large particles, approximately 200 microns to 5 mm, of uniform size, such as Thixomolding.TM. of alloys, and other applications in ceramics, ceramic metal combinations, metals and metal alloys provide a demand which is presently not fully satisfied. Current practices for the formation of large particles are expensive, and do not provide the level of shape, uniformity and purity demanded.
A common prior art practice is disclosed in U.S. Pat. No. 4,428,894 by Bienvenu issued in 1984 in the name of Extramet. A jet of molten metal is passed through a vibrating orifice. Drops formed fall from the orifice under the action of gravity through an inert gas atmosphere at a cooling temperature. If particles larger than one millimeter in diameter are to solidify to a point where sphericity is maintained after impacting the bottom, an extremely tall cooling tower is required. This cooling tower method also causes the droplets to pass through the inert atmosphere at high relative velocity, approximately 20 meters per second. In a technique called "double fluid atomization" a high pressure gas flow is introduced causing an even higher relative velocity. High relative velocity, it has been found, distorts the spherical shape of the droplets. In addition impact with the chamber walls prior to solidification, or impact with the bottom of the cooling tower if a quench liquid is not used, flattens the particles unless the cooling tower is sufficiently tall. When quench liquids are used to remove significant latent heat, droplets that are still liquid or semi-solid can lose their spherical shape upon impact with the quench liquid. Thus even with a quench liquid, residence time in a cooling tower must still be maximized in order to permit droplets to cool sufficiently to reduce deformation.
Other factors which adversely affect particle shape include agglomeration with other droplets prior to solidification which affects the shape and size distribution of particles. Since individual droplets spheroidize from a ligand shape caused by the breakup of a liquid stream, a particular problem in the case of high melting point materials is that solidification can occur prior to spheroidization of the droplet causing irregularly shaped particles. A further problem is associated with surface oxidation. Oxides normally have a much higher melting point, and for skin-forming alloys like aluminum, this layer forms almost immediately and can make spheroidization impossible. Oxidation, it is known, can be reduced by providing an inert gas atmosphere within the cooling tower. Since a cooling tower can be 20 meters high, circulating a cooling inert atmosphere throughout can be quite expensive.
Control of particle size distribution is also important to particle production. Uniform particles are easier to model in applications such as Thixomolding.TM. or alloying. Use of a Rayleigh wave disturbance to impart predetermined, vibration induced break up of an unstable liquid stream has been used extensively to control the formation of uniform droplets.
Most metals and alloys are more reactive in the molten state than in the solid state. As a result, it is desirable to make the time a droplet spends in molten state as short as possible. Commonly, in prior art practices, this is accomplished by quenching the droplets in a fluid with a high heat transfer coefficient, as soon as spheroidization has occurred. However, often an undesired reaction occurs between the particle surface and the quench liquid. For a highly reactive alloy, such as magnesium, this would cause unacceptable contamination. It is necessary for reactive metals to maximize the time spent cooling in an inert gas, that is the residence time, before removing the bulk of latent heat with a quench liquid, otherwise particles may be contaminated through chemical bonding with other materials. Thus, for large particles holding significant latent heat, maximizing cooling time requires a very tall, and expensive, cooling tower.
U.S. Pat. No. 4,871,489 by Ketcham, issued to Coming Incorporated in 1989, discloses the use of an inverted apparatus produced by Thermo Systems Incorporated for the production of metal oxide precursors. This apparatus is designed for the production of very fine particles, having a diameter of about 8.5 microns and not larger than 50 microns. Fluid is forced though a thin perforated plate to form a plurality of fluid streams. Oscillation of the plate is applied in the direction of the fluid flow to break up uniform droplets. The droplets are entrained in the flow of a dispersion medium which dries and removes the light particles. However, this device is not adequate for the formation of larger particles which have greater latent heat and kinetic energy. Sufficient cooling would not occur as particles are entrained in the dispersion fluid. The flow of dispersion fluid necessary would be rapid to lift the heavy particles from the chamber, which would adversely affect the particle shape. In addition, the greater latent heat and longer cooling time would lead to increased particle agglomeration as still molten particles contact one another in the dispersion flow. This patent does not teach a method for increasing the residence time for the formation of large uniform and spherical particles.
It is desired to provide relatively large uniform and spherical particles, without reactive contaminants. A more economical apparatus is needed, suitable for highly reactive materials which would reduce distortion of particle size and shape. It is proposed to provide an inverted cooling chamber that releases a molten stream at or near the bottom to launch large particles on a parabolic trajectory having an upward and downward path. This provides a longer cooling time in a controlled atmosphere at low relative velocity without the large cooling tower currently required by the prior art.